Wild macaques who are bystanders to fights within their group exploit the losers for grooming favours, new research has shown.
The findings, published in the journal Animal Behaviour, reveal previously unknown details about the important function observing others' aggressive behaviour serves in primate society, and may even help to explain why humans often hold a fascination with watching fights.
The study by Dr Bonaventura Majolo from the University of Lincoln, UK, and Dr Richard McFarland from the University of Witwatersrand, Johannesburg, South Africa, is the first of its kind to fully scrutinise 'bystander affiliation' in wild Barbary macaques (Macaca sylvanus).
Bystander affiliation describes a range of friendly social interactions, such as grooming, shared by victims and witnesses following aggressive interactions in animal societies.
Reconciliation between former adversaries after confrontations is a common and well-studied aspect of primate society but the nature of bystander affiliation is less well-understood.
It had been assumed the behaviour serves largely as consolation for the victim, alleviating anxiety felt after a defeat. However, the latest study suggests that bystanders may have more selfish motivations.
The team found that in most instances, it is bystanders who initiate contact with victims and when grooming does result, bystanders reap the benefits for longer.
They concluded that rather than offering consolation or protection, bystanders tend to exploit the vulnerable position of defeated individuals in order to elicit grooming. Grooming is not just a socially important behaviour in primate society; it also confers selective advantages, such as the removal of parasites.
Dr Majolo, from the School of Psychology at the University of Lincoln, said: "One of the reasons why we humans are so interested in watching aggressive interactions may be that these conflicts provide a wealth of important information about others in our group. There should therefore have been strong selective pressure on our ancestors to pay attention to such events.
"Our observation of grooming exploitation among wild Barbary macaques would support the theory that observing aggressive interactions between group companions can have a range of social benefits and functions."
Barbary macaques are a deeply ancestral species of macaque and the last of their kind remaining in Africa. An IUCN Red List Species, most wild populations are found in the Middle Atlas Mountains range of North Africa.
Drs Majolo and McFarland are part of an international team of academics which has established a successful field site near the city of Azrou, Morocco, where they are observing several groups of Barbary macaque in the wild comprising more than 50 individuals.
Their latest findings are compatible with their recent study which provided rare evidence of 'coercion' among primates in the wild. They found dominant Barbary macaques appeared to provoke confrontations with the sole aim of eliciting grooming from subordinates.
Acts of aggression were defined by the researchers as attempts by one animal to threaten, lunge, chase, slap, grab or bite another. Protagonists were labelled as aggressor or victim, depending on who initiated the conflict. Bystanders were also identified.
The researchers recorded 191 instances of 'post conflict' behaviour involving a victim of aggression. Around a quarter (24%) of those were examples of bystander affiliation. Of these cases, almost half (49%) were initiated by a bystander, 38% by a victim and the remaining 13% were considered to be mutual. In other words, it was more common for bystanders to approach victims immediately after aggression than the other way round. When grooming occurred, bystanders received significantly more grooming than victims.
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McFarland R & Majolo B (2012). 'The occurrence and benefits of bystander affiliation in wild Barbary macaques, Macaca sylvanus. ' Animal Behaviour 84 (2012) 583-591. DOI: 10.1016/j.anbehav.2012.06.010